Final Keyword in C# – sealed with const and readonly

In java there is keyword “final“, which is used to avoid overloading / inheritance of method / class respectively.

In c# there is no keyword like “final” but the same thing is achieved by keyword “sealed“.

A class which is marked by keyword sealed cannot be inherited.

If you have ever noticed, structs are sealed. You cannot derive a class from a struct.

Example:

namespace OOPS_Concept
{
    sealed class SealedClassDemo
    {
        public void test1()
        {
            Console.WriteLine("Method Test1");
        }
    }

    class childclass : SealedClassDemo
    {
    }
}

Above code will generate an error saying that class cannot be inherited.

Sealed Method:

In C#, Methods cannot be “sealed” directly. Methods of only derived class can be made sealed with keyword sealed and override.

namespace OOPS_Concept
{
    class SealedMethodDemo
    {
        public virtual void Method1()
        {
            Console.Write("Base class Method1");
        }
    }

    class ChildClass : SealedMethodDemo
    {
        public sealed override void Method1()
        {
            Console.Write("Derived class Method1");
        }
    }
}

One of the best usage of sealed classes is when you have a class with static members. For example, the Pens and Brushes classes of the System.Drawing namespace.

readonly:

The readonly keyword is a modifier that you can use on fields. When a field declaration includes a readonly modifier, assignments to the fields introduced by the declaration can only occur as part of the declaration or in a constructor in the same class.

refer this MSDN document to read more on keyword “readonly” in C#

const:

The const keyword is used to modify a declaration of a field or local variable. It specifies that the value of the field or the local variable cannot be modified. A constant declaration introduces one or more constants of a given type. The declaration takes the form:

public const double x = 1.0, y = 2.0, z = 3.0;

Note : The readonly keyword is different from the const keyword. A const field can only be initialized at the declaration of the field. A readonly field can be initialized either at the declaration or in a constructor. Therefore, readonly fields can have different values depending on the constructor used. Also, while a const field is a compile-time constant, the readonly field can be used for runtime constants, as in the following example:

public static readonly uint l1 = (uint) DateTime.Now.Ticks;

MSDN article on const keyword